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506                               EXTINCT   RODENTS

to note that some of the extinct genera were much larger that
recent forms. At present, HydrocJioerus Is the biggest Rodent
but the genus Megcwiys from the Pampas formation of Argentina
was " nearly as large as an ox." The wider range of genera ir
the past is illustrated by JEfystrix, which, now an Old-Worlc
form, is represented by remains in the Miocene and Pliocene oj

It is a significant fact that of living genera Sciurus is the
oldest; for it has been pointed out that In a number of features
the Squirrels are among the most primitive of Rodents. The
jzygomatic arch Is slender, and has thus not acquired the specialisa-
tion that is to "be found in that part of the skull in other Rodents;
moreover, the " jugal bone Is not supported by any process from
the maxilla exactly as in the primitive Ungulata." The feet,
too, are unspecialised, though that is the case with many other
genera. It may also be pointed out that the teeth bear not a
little likeness to those of Ornithorhynchus in their multifcuber-
culate character.

Some few fossil forms have already been dealt with in the
preceding pages.

The two genera Castoroides and -Amblyrhiza, from the Pleisto-
cene of North America and the "West Indies, are usually regarded
as forming a family. The skull of the former genus indicates an
animal of the size of a Bear. It is compared to that of Castor,
but it has a wide Infra-orbital foramen. The teeth are four in
each jaw, and are formed of three to five lamellae; the Incisors of
this animal are powerful but short. A-mblyrtiiza, on the other
hand, has long Incisors which are longitudinally grooved anteriorly.
It has a free fibula. This latter as well as other characters have
led Tullberg to remove It from association with Castoroides.

Order X.     TILLOBONT1A.
This group of Eocene mammals is to be defined by a number
of characters, of which the more important are the following :
The Incisors are enlarged, grow from persistent pulps, and are
coated with enamel upon the outer surface only ; they are those
of fche second pair only, the first and third having disappeared
or become small. The canines are reduced in the later forms-